1. Field of the Invention
The present invention relates to a method of producing a slurry of microcapsules, particularly a slurry of microcapsules having wall membranes composed of an amino resin.
2. Related Art Statement
Microcapsules were developed and used originally as materials for non-carbon papers by NCR Corporation in the fist half era of 1950, so that they have a past record of more than 30 years in the technical field. They function to tentatively protect and stabilize the core materials which per se are originally unstable, and discharge the core materials gradually or instantaneously in case of necessity. Recently, techniques for producing microcapsules have advanced remarkably, so that microcapsules (sometimes abbreviated as "capsules", hereinafter) are widely used in various fields such as, medicines, cosmetics,pesticides, perfumes, foods, dyes, etc., in addition to non-carbon papers.
For producing microcapsules, physical methods, mechanical methods, physical chemical methods, and chemical methods, are known and selectively used depending on the use to which the microcapsules will be put.
By using the physical methods or the mechanical methods, microcapsules of large sizes and coarse wall membrane are produced, so that their use is very narrowly restricted.
In contrast, the physical chemical methods and the chemical methods can arbitrarily and easily control the size of the microcapsules down to a small size of a few .mu.m with a dense wall membrane, so that such microcapsules are widely used in many use fields.
Among the physical chemical methods, a coacervation method using gelatine as a wall membrane material is known. It is now used most widely, however, it uses a natural product, gelatine, as the wall membrane material, so that it has drawbacks in that the cost is expensive, the microcapsules are suceptible to attacked by microorganisms, have poor resistance to water, microcapsules slurries of high concentrations are difficult to obtain, and the process steps are rather complicated.
The chemical methods include the interfacial polymerization method and the in-situ polymerization method. In the interfacial polymerization method, a hydrophobic monomer and a hydrophilic monomer are copolymerized at an interface between a hydrophobic core material and a water phase to form microcapsules wall membranes of polyamide, epoxy resin, polyurethane, or polyurea, etc. In the in-situ polymerization method, a hydrophobic monomer and a hydrophilic monomer are copolymerized at either of the hydrophobic core materials or the water phase to form wall membranes of amino resin, etc., around the core materials.
As disclosed, for example, in Japanese Laid-open Patent Application Nos. 49-25,822, 52-48,599, 54-91,980, 56-78,182, and 56-88,954etc., the interfacial polymerization method uses a reactive or noxious material, such as isocyanate, acid-chloride, or epoxy compound, etc., as the wall membrane material, so that it has many drawbacks in that the polymerization reaction is hard to control, a material having an active hydrogen can not be used as the core material, and the wall membrane material per se is expensive. In contrast, the in-situ polymerization method can use almost any of synthetic resins, such as acrylate resin, amino resin, polyester resin, and the like, as the wall membrane material for microcapsules, and has advantages that the amino resin is cheap and easily available, the polymerization reaction of the amino resin can be effected in a comparatively short time, and a special catalyst is not necessary for the reaction. Thus, the in-situ polymerization method is easy to perform as compared with the interfacial polymerization method, so that it is a more practical method than the latter, and many inventions regarding the in-situ polymerization method have been developed and filed as patent applications, such as Japanese Patent Nos. 44-3,495, 47-23,165 and 45- 16,949, and Japanese Laid-open Patent Application Nos. 53-84,881 and 54-49,984. However, though the method can provide microcapsules having wall membranes, it has drawbacks in that the wall membranes exhibiting superior water resistance are coarse, emulsification and dispersion of the core material are not sufficiently effected, and further the polycondensation product can hardly be deposited efficiently and stably around the core material.
Methods of producing microcapsules by the in-situ polymerization method wherein a melamineformaldehyde resin or an ureaformaldehyde resin is used as the wall membrane material are already known as described in Japanese Patent Nos. 37-12,380, 44-3,495, and 47-23,165. However, as mentioned above, these methods can hardly deposit the polycondensation product efficiently and stably around the hydrophobic core material, emulsification and dispersion of the core materials are not sufficiently effected.
In order to overcome the aforementioned drawbacks, Japanese Patent No. 54-16,949, and Japanese Laid-open Patent Application Nos. 53-84,881, 53-84,882, and 53-84,883 disclose use of an anionic polyelectrolyte, such as ethylene/maleic anhydride copolymer, methylvinylether/maleic anhydride copolymer, or polyacrylic acid, etc., as a reaction system modifier. According to these methods, the emulsifying and dispersing properties of the hydrophobic core material is improved, strength and density of the wall membranes are enhanced, and a slurry containing strong microcapsules can be obtained in a short period of time. However, the viscosity of the obtained microcapsules slurry is high, and a considerably long time is required for dissolving the particularly effective ethylene/maleic anhydride copolymer, or methylvinylether/maleic anhydride copolymer.
Japanese Laid-open Patent Application Nos. 54-49,984, 55-47,139, and 55-15,660, disclose use of a styrene/maleic anhydride copolymer, or a mixture thereof with vinylacetate/maleic anhydride copolymer or ethylene/maleic anhydride copolymer, as the reaction system modifier. According to these methods, the emulsifying and dispersing property of the core materials are further improved to form a stable and low viscosity slurry containing microcapsules of high strength. However, styrene/maleic anhydride copolymer has drawbacks in that it precipitates at a low pH range so that it can not be used for ureaformaldehyde series wall membrane materials which are in many cases polycondensated condensated at a low pH range, and it can not be used in the process of removing residual formaldehyde for removing unreacted residual formaldehyde at a low pH range.
Japanese Laid-open Patent Application No. 56-51,238 discloses use of styrene sulfonic acid series polymers, such as acrylic acid/styrene sulfonic acid copolymer, polystyrene sulfonic acid, or the like which is an anionic polyelectrolyte, as the reaction system modifier. The styrene sulfonic acid series polymers are stable even at a low pH range, so that the polycondensation reaction in the ureaformaldehyde system or the treatment of removing the residual formaldehyde at a low pH range becomes possible. However, the styrene sulfonic acid series polymers in the melamine/formaldehyde system generates much foam at the time of electrolyzing resulting in troublesome and poor workability, and the styrene sulfonic acid series polymers in the urea/formaldehyde system induces agglomeration of the whole system even with only a small change in the reaction conditions.
The inventors have disclosed previously in Japanese Laid-open Patent Application No. 60-238,140 a novel method of producing microcapsules which uses a copolymer of acrylic acid/acrylamide/acrylonitrile as the reaction system modifier. According to this method, a slurry of microcapsules of good quality can be obtained in a short reaction time by using a ternary copolymer of acrylic acid/acrylamide/acrylonitrile as the modifier.
Nevertheless, recently a slurry of microcapsules of a better quality, a lower viscosity, and a higher microcapsules concentration, has been earnestly desired in accordance with the advanced progress of technology.